An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent (EL) unit sandwiched between the anode and the cathode. The organic EL unit includes at least a hole-transporting layer (HTL), a light-emitting layer (LEL), and an electron-transporting layer (ETL). OLEDs are attractive because of their low drive voltage, high luminance, wide viewing-angle and capability for full color displays and for other applications. Tang et al. described this multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
OLEDs can emit different colors, such as red, green, blue, or white, depending on the emitting property of its LEL. Recently, there is an increasing demand for white light-emitting OLEDs to be incorporated into various applications, such as a solid-state lighting source, color display, or a full color display. By white light emission, it is meant that an OLED emits sufficiently broad light throughout the visible spectrum so that such light can be used in conjunction with filters to produce a full color display. In the case of OLED displays, the use of white OLEDs with color filters provides a simpler manufacturing process than an OLED having separately patterned red, green, and blue emitters. This can result in higher throughput, increased yield, and cost savings. White OLEDs have been reported in the prior art, such as reported by Kido et al. in Applied Physics Letters, 64, 815 (1994), J. Shi et al. in U.S. Pat. No. 5,683,823, Sato et al. in JP 07-142169, Deshpande et al. in Applied Physics Letters, 75, 888 (1999), and Tokito, et al. in Applied Physics Letters, 83, 2459 (2003).
In order to achieve white light emission from an OLED, it is usually required that more than one type of molecule has to be excited, because each type of molecule typically emits light with a relatively narrow spectrum under normal conditions. A light-emitting layer having a host material and one or more luminescent dopant(s) can achieve light emission from both the host and the dopant(s) resulting in a broadband emission in the visible spectrum if the energy transfer from the host material to the dopant(s) is incomplete. To achieve a white OLED having a single light-emitting layer, the concentrations of light-emitting dopants need to be carefully controlled. This produces manufacturing difficulties. A white OLED having two or more light-emitting layers can have better color as well as better luminance efficiency than a device with one light, and the dopant concentration variability tolerance is higher. It has also been found that white OLEDs having two light-emitting layers are typically more stable than OLEDs having a single light-emitting layer. However, it is difficult to achieve light emission with strong intensity in the red, green, and blue portions of the spectrum. A white OLED with two light-emitting layers typically has two intensive emission peaks. It is known to use a third light-emitting layer to provide a third intensive emission peak.
For certain applications, e.g. televisions, color reproduction is very important. Not only is it important to have effective efficiency, but the color purity of light after passing through a filter needs to be excellent. This is achieved through the use of very narrow band pass color filters. Unfortunately, this wastes a large portion of the emitted light resulting in very low power efficiency. Typically, color filters are designed to have relatively broadband pass. Quite commonly, the band pass of color filters for display applications overlap in certain portions of the spectrum. For example, the blue and green filters can both permit some light in the blue-green portion. This greatly enhances the brightness of light passing through the filter, but it also can lead to unwanted color contamination resulting in desaturated primary colors.
Recently, a tandem OLED structure (sometimes called a stacked OLED or a cascaded OLED) has been disclosed by Jones et al. in U.S. Pat. No. 6,337,492, Tanaka et al. in U.S. Pat. No. 6,107,734, Kido et al. in JP Patent Publication 2003/045676A and in U.S. Patent Publication 2003/0189401 A1, and Liao et al. in U.S. Pat. No. 6,717,358 and U.S. Patent Application Publication 2003/0170491 A1, the disclosures of which are herein incorporated by reference. This tandem OLED is fabricated by stacking several individual OLED units vertically and driving the stack using a single power source. The advantage is that luminance efficiency, lifetime, or both are increased. However, the tandem structure increases the driving voltage approximately in proportion to the number of OLED units stacked together.
Matsumoto and Kido et al. reported in SID 03 Digest, 979 (2003) that a tandem white OLED is constructed by connecting a greenish blue EL unit and an orange EL unit in the device, and white light emission is achieved by driving this device with a single power source. Although luminance efficiency is increased, this tandem white OLED device has weaker green and red color components in the spectrum. In U.S. Patent Application Publication 2003/0170491 A1, Liao et al. describe a tandem white OLED structure by connecting a red EL unit, a green EL unit, and a blue EL unit in series within the device. When the tandem white OLED is driven by a single power source, white light emission is formed by spectral combination from the red, green, and blue EL units. Although color emission and luminance efficiency is improved, this tandem white OLED cannot be made with less than three EL units, implying that it requires a drive voltage at least 3 times as high as that of a conventional OLED. In addition, it is known that blue light-emitting OLED units are not as stable as white light-emitting units. U.S. Pat. No. 6,903,378 discloses a tandem OLED having two white light-emitting EL units and color filters. However, there is little disclosure about how to optimize these EL units to produce improved performance.
A need exists for displays that are simple to make, but also have effective color gamut and high efficiency.